And indeed, the WSGI daemon definition is included twice. I see two options here:

a) Since I want to enforce SSL for this site (haven't figured out how to do that -- is it possible using ispconfig only?), I don't need the WSGI Daemon definition in the Port 80 Vhost. But how could I do that in ispconfig?

b) The parameter I give to the WSGIDaemonProcess directive should differ between the port 80 and port 443 vhosts. Is this possible?